The treatment and/or prevention of pressure ulcers are serious and expensive issues in the health care industry. Pressure ulcer development is related, in part, to the accumulation of heat and perspiration on the skin. Heat and moisture increase skin susceptibility to the damaging effects of pressure and shear and decrease the resiliency of the epidermis to external forces. Ongoing compressive forces on skin tissues are known to promote ischemia with subsequent development of pressure ulcers. Therefore, controlling the microclimate of the skin and providing a quality patient support system appear to be necessary to prevent pressure ulcers.
Currently, low-air-loss (LAL) mattress systems are the most prevalent tools used for pressure ulcer treatment and prevention. LAL mattress systems were developed and are used in the belief that they help to control the microclimate of the skin. These systems have been found to be highly effective in treating and/or preventing pressure ulcers.
Low-air-loss broadly refers to a system comprising a mattress casing, a vapor permeable coverlet with or without lofting or cushioning material, and an air delivery system to move air under the coverlet and, in some cases, to leak through the coverlet. Some LAL mattress systems function as integral parts of patient support systems; whereas, others are not actively coupled.
LAL mattresses typically include a foundation of a series of interconnected air cells that allow air to flow through and exit the mattress. Other common elements include an adjustable pump that can maintain air inflation of the air cells. In addition to the mattress, the LAL mattress system also includes the coverlet (waterproof and/or vapor permeable), and coverlet lofting material (e.g., quilted polyester fabric batting) that attach over the mattress. The coverlet is typically made of a material(s) that is permeable to moisture, is impermeable to bacteria, and is waterproof. Coverlets also function to prevent excessive loss of body heat, have high moisture vapor permeability to minimize/prevent the accumulation of perspiration on the skin, and have high air porosity for removal of excessive body heat through a continuous airflow provided by the LAL mattress. Together, the LAL mattress and the coverlet form the LAL mattress system.
The LAL mattress can further include a fabric cover over the foundation (i.e., the air cells). In some cases, this fabric cover is formed from a Goretex™ fabric. The Goretex™ fabric is liquid impermeable and has significantly higher air-permeable and vapor permeable characteristics as compared to urethane-backed nylon materials used in other mattresses. The Goretex™ fabric moisture vapor transfer characteristics helps to prevent the formation or speed up healing of pressure ulcers in patients by reducing the amount of moisture buildup on the skin and by helping to keep patients cooler by allowing body heat to more easily escape.
LAL mattress systems are intended to remove or reduce the amount of perspiration on the skin of the patient, the evaporation of which provides cooling to control skin microclimate. The two mechanisms that are used by the LAL system to remove moisture and heat are diffusive evaporation and convective evaporation. In diffusive evaporation, moisture is evaporated through and under the coverlet to cool the skin without the need to physically blow air on the patient to keep the skin cool and dry. On the other hand, convective evaporation relies on moving air directly against the skin to evaporate perspiration. Both mechanisms rely on removing moisture away from the patient and the mattress. Some LAL systems incorporate features of both convective and diffusion methods. Either mechanism, however, depends on the LAL mattress system not only drawing moisture away from the patient, but also removing that moisture from the mattress system itself. If this is not accomplished, the patient lies in a damp environment that contributes to skin breakdown and susceptibility to bacterial growth.
Evaporation of moisture off the skin can result in significant cooling of body temperature. In LAL mattress systems low humidity air circulates under the patient's support cover, increasing evaporation and cooling. Although the removal of perspiration on the skin is significant, there are no absolute guidelines on the amount of moisture that should be removed or the decrease in body temperature that should result from the use of an LAL mattress system. Most LAL manufactures agree that the airflow of the system should be at least the amount needed to remove perspiration of an average person at rest in a moderate climate. A typical inactive patient with a body temperature of 37 degrees C. perspires about 600 g/day in a continuous manner. An average prostrate patient provides a mattress pressure loading of approximately 10 mm Hg over the torso area.
Without firm guidelines, physicians must order support systems (e.g., LAL mattress systems) based on cost, claims of the suppliers, and prior experience. Therefore, to improve the selection process of support systems and to further advance the design these systems, measurable parameters that accurately reflect moisture transport from a support systems need to be established and standardized. There are a variety of different designs for LAL mattress and other support systems. But for the variety of designs, there is yet to be an acceptable reproducible standard on which to base their performance and/or assess the anticipated clinical effect of the support systems.
A moisture vapor transfer (MVT) rate, or flux, is one parameter that is currently measured in assessing the performance of LAL mattress and other support systems. The MVT rate can be measured as grams (of human perspiration) per (Meter2) per (hour). Typically, the MVT rate is measured from a support system using a batch process. In this batch process, a fixed amount of a test fluid (e.g., water) is adsorbed in a test media (e.g., a non-woven towel) and pressure and heat are applied to simulate a patient's weight, heat, and perspiration load at the patient-mattress interface of the support system. The MVT rate is then determined by making weight measurements of moisture loss from the test media over a set time period, or measurement of the time of transfer of a known volume of liquid.
One issue with the above technique, however, is that the MVT rate measured using this system is proportional to the amount of moisture in the test media. For example, the MVT rate will be higher at the beginning of the test when the most moisture is present and lower as the test media dries out, and would go to zero if the test media is allowed to completely dry out. During this batch drying process, the moistened test media is exposed to an operating LAL mattress or other support system into which the moisture evaporates. This type of test takes place under dynamic or non steady-state conditions: the test media is charged with the moisture, which remains in the support system until dry. The weight of the test media is then measured as a function of time at the end of the drying process.